In an electrostatic copying machine, including a copying unit comprising a photosensitive drum and a developing device, it is sometimes necessary to pull the copying unit out of the housing in order to perform maintenance work as well as to remove jammed recording sheets or to resupply the developer. Power is transmitted from a driving shaft in the housing to a driven shaft in the copying unit when the copying unit is loaded in the housing and the transmission of power from the driving shaft to the driven shaft is prevented when the copying unit is pulled out of the housing.
In a conventional power transmitting device, the driven shaft is a spline shaft. A boss, namely, a spur gear corresponding in configuration to the spline, is mounted on the driven shaft and is urged toward the driving shaft by a spring. The driving shaft has a spur gear which is engaged with the spur gear of the driven shaft.
A sprocket wheel is fixedly mounted on the driving shaft, and a chain is laid over the sprocket wheel. The chain is driven by a power source such as an electric motor. Tension is maintained on the chain by the sprocket wheel which is engaged with the chain and is energized by a spring. The gear of the driven shaft and the gear of the driving shaft are disengaged from each other when the copying unit is pulled out of the housing. At the same time, the driving shaft becomes freely rotatable and the driving shaft and the gear coupled to the driving shaft are slightly turned (angularly displaced) by the tension given to the chain. This angular displacement makes it difficult for the gears to engage when the copying unit is pushed back in and sometimes causes it to be impossible to transmit power.